This invention relates to an improved process for removing tin and tin-lead alloy coatings from copper substrates and is more particularly concerned with a two step process for removing said coatings from a copper substrate without removing any significant amount of the copper substrate itself.
There is a wide range of situations in which it is necessary to remove a layer of tin or tin-lead alloys such as solder from a copper substrate. Illustratively, the rejects which sometimes arise in the electroplating of copper components with a layer often have to be stripped of the layer in order to recover and re-use the copper substrate.
Another situation is that which involves removal of tin or tin-lead alloy etch resists from copper substrates. A particular example of this stripping tin-lead etch resists employed in the fabrication of solder mask on bare copper (SMOBC) type printed circuit boards. Thus, in such fabrication, tin-lead alloys are plated onto a copper substrate in a predetermined pattern to delineate the printed circuit and to serve as an etch resist in subsequent etching of the copper. After the etching is completed the presence of the tin-lead in the through holes of the board and their surrounding pads is desirable to improve solderability. However, in some areas, it is preferable to remove the tin-lead especially from sliding contact fingers where these are to be plated with nickel and/or gold to improve conductivity. Removal of tin-lead from circuit traces is also desirable prior to covering the circuit with solder mask.
The stripper compositions hitherto employed in the art to remove tin and tin-lead alloys from copper substrates generally fall into one of three classes. One class is based on nitro-substituted aromatic compounds such as nitro-substituted aromatic sulfonic acids and salts thereof as the principal, or one of the principal, active ingredients. Representative of this class are the compositions disclosed in U.S. Pat. Nos. 3,677,949; 4,004,956; 4,397,153; and 4,439,338. A second class of compositions is based on hydrogen peroxide as the principal active ingredient in acidic medium. Representative of this class are the compositions disclosed in U.S. Pat. Nos. 3,926,699; 3,990,982; 4,297,257; 4,306,933; 4,374,744; and 4,424,097. A third class comprises a mixture of an alkali metal hydroxide and a chlorite in concentrated aqueous solution. However, this type of stripper composition is significantly slower (approximately 10 times slower) than the first class of stripper mentioned above.
In the case of the first two classes of composition mentioned above there is present a combination of an oxidant for the tin or tin-lead and an acceptor for the cation or cations so oxidized. The oxidant is the nitro-substituted aromatic compound or the hydrogen peroxide and the acceptor is an acid as fluoboric, acetic or like aliphatic acid or aromatic acids.
The hydrogen peroxide based compositions have a number of disadvantages. Thus they are too unstable on storage to be provided in ready-to-use form and have to be prepared just prior to use. Further, the reaction involved in the stripping using the hydrogen peroxide based compositions is highly exothermic and the tin-lead alloy turns black. A white deposit is formed on the substrate and significant attack and removal of copper from the substrate takes place.
Of the various classes of stripper the nitro-aromatic compound-based compositions are generally preferred since they have much greater stability and are less aggressive towards the copper substrate than the hydrogen peroxide based compositions. However, they suffer the drawback that a residual thin layer of tin, due either to incomplete stripping or redeposition of tin from the stripping bath, is difficult to remove without excessive attack on the copper substrate itself. Brindisi et al, U.S. Pat. No. 3,677,949 sought to overcome the problem of tin redeposition by adding a thiourea to the stripper bath which comprised a nitro-aromatic sulfonic acid or like compound and a fluoboric acid. It is found in practice, however, that such formulations are not stable on storage and the thiourea is precipitated in orange-brown crystalline form. Czaja U.S. Pat. No. 4,397,753 reported finding a whitish residue on the copper substrate after stripping using a formulation prepared as taught by Brindisi et al and recommended the addition of a hydroxyphenol to a nitro-substituted aromatic sulfonic acid/fluoboric acid mixture.
It has been found that both the Brindisi et al and the Czaja compositions will strip tin-lead alloys from copper substrates in a clean manner when the stripper solution is relatively fresh and the dissolved tin concentration is less than about 1 gram per liter. However, in actual practice, this level is rapidly reached and thereafter re-deposition of tin from the solution apparently occurs.
The present invention is based on the finding that the thin residual layer of tin remaining on the copper substrate after stripping using the prior stripping compositions can be removed readily without employing rigorous stripping conditions and without sacrifice of any significant amount of copper from the underlying substrate.